The present invention relates to a method and apparatus for use in dilating occluded blood vessels and for measuring the degree of dilation of the occlusions within these vessels. Prior to the present invention these two objectives were attainable, as a result of the teachings set forth in the above-identified copending patent applications, by the use of two catheters, one having a balloon element to dilate the occlusion and the other having a calibrator element to measure the widened lumen of the occluded segment of artery. This could result in the repeated insertion and removal of catheters into and out of arteries until the sizes of the enlarged passages of the occluded segments of the arteries were of acceptable dimensions. The heavier the traffic of catheters within blood vessels the greater is the risk that material may be accidentally dislodged therefrom with possible consequent blockage elsewhere in the blood circulation system.